Introduction
Abdominal aortic aneurysm (AAA) is defined as abnormal progressive dilatation of abdominal
aorta.[1] The objective of this review article is to understand current concept of risk factors
for developing AAA, level-1 evidence for endovascular aortic aneurysm repair (EVAR),
selection criteria for a patient for EVAR, currently used grafts, principles of EVAR
procedure, descriptions of advance technique with AAA for poor access routes and hostile
neck, evidence for endovascular AAA for ruptured abdominal aortic aneurysm (rAAA),
and common complications associated with this procedure.
Risk Factors for Abdominal Aortic Aneurysm
Among the various variable risk factors (hypertension, inherited diseases, and connective
tissue disorders), smoking and increasing age appears to be the most important for
development of AAA. In a population-based study of 6,386 men and women aged 25 to
80 years in Tromo, Norway, an aneurysm was present in 263 (8.9%) men and 7 (2.2%)
women (p < 0.001). The prevalence increases with increase in age. Person who had smoked for
more than 40 years has an odd ratio of 8.0 of having AAA (Ullery et al).[2] AAA has life-threatening complication of rupture. Fifty percent of patients with
ruptured AAA are not able to reach hospital and 30 to 50% have in-hospital mortality
even after an emergency repair. Mortality rate is much less if the aneurysm is repaired
electively before aneurysm rupture.
EVAR is a minimally invasive method of treating AAA. This was first described by Parodi
in 1991 and involves placement of a stent-graft across the aneurysm to exclude it
from arterial circulation. It obviates the need for laparotomy and aortic cross-clamping,
as needed in open repair. EVAR has revolutionized aortic aneurysm management. Currently,
in most vascular centers across the world, majority of AAAs are treated by EVAR.
Repair of AAA is taken when its diameter reaches to 5.5 cm or more. It is also indicated
if the size of aneurysm increases to greater than 0.5 cm within 6 months.[3]
Randomized Controlled Trials for Endovascular Aortic Aneurysm Repair
Many randomized controlled trials (EVAR-I, Dutch Randomized Endovascular Aneurysm
Management [DREAM] trial, open versus endovascular repair of abdominal aortic aneurysm
[OVER] trail, and ACE [Aneurysme de L aorte abdominale: Chirugie versus Endoprosthese]
trial) had shown that EVAR is associated with significant reduced perioperative mortality
compared with open repair,[4] and it is also associated with shorter operating time, reduced in-hospital stay,
and decreased blood loss as compared with open repair.[5]
In EVAR trial, men and women aged ≥60 years with an aneurysm of ≥5.5 cm (as identified
by computed tomography [CT] scanning), anatomically suitable and fit for operating
room (OR) were randomly assigned 1:1 to either EVAR (n = 626) or OR (n = 626). At 0 to 6 months after randomization, patients in the EVAR group have a lower
mortality (adjusted hazard ratio [HR] = 0.61, 95% confidence interval [CI]: 0.37–1.02
for total mortality; HR = 0.47, 95% CI: 0.23–0.93 for aneurysm related mortality,
p = 0.036). Over a mean of 12.7 years (standard deviation = 1.5 years, maximum = 15.8
years), 9.3 deaths per 100 person-years recorded in the EVAR group and 8.9 death per
100 persons.
The ACE trial compared mortality and major adverse events after EVAR and open surgical
repair (OSR) in patients with AAA anatomically suitable for EVAR and at low or intermediate
risk for open surgery. A total of 316 patients of >5 cm aneurysms were randomized
in institutions with proven expertise for both treatments. A total of 316 patients
of >5 cm aneurysms were randomized in institutions with proven expertise for both
treatment: 219 were available for analysis and 149 were assigned to OSR and 150 to
EVAR. Patients were monitored for 5 years after treatment. Statistical analysis was
by intention to treat. With median follow-up of 3 years (range: 0–4.8 years), there
was no difference in the cumulative survival free of death, or major events rates
between OSR and EVAR of 95.9 ± 1.6 versus 93.2 ± 2.1%.
Patel et al assessed term efficiency of EVAR against open repair in patients deemed
fit and suitable for both procedure (EVAR trial 1; EVAR) against no intervention in
patients unfit for OR (EVAR trial 2; EVAR 2). EVAR has an early survival benefit but
an inferior late survival benefit compared with OR which needs to address by life-long
surveillance of EVAR and reintervention if necessary. EVAR does not prolong life in
patients unfit for OR (Patel et al). They highlighted limitations that devised used
were implanted between 1999 and 2004. New devices might have better results.
Selection Criteria for Standard Endovascular Aortic Aneurysm Repair
CT angiography (CTA) is the main imaging modality for preoperative planning.[6] Certain anatomical standards must be fulfilled for successful EVAR deployment. To
achieve adequate image quality, standard slice thickness of ≤1 mm is taken. Workstation
image processing provides accurate diameter, angle, and curvilinear length measurements.
Four anatomical areas are assessed when considering for suitability for EVAR: proximal
fixation site, distal fixation sites, aneurysmal morphology, and distal vessel evaluation.
Proximal Aortic Neck
It is the proximal fixation site for stent-graft.[7] Its length, angle, diameter, shape, presence of calcification, or thrombus in it,
all have effect on the EVAR outcome. Aortic neck is measured from lowest renal artery
to the start of aneurysm. Aortic neck less than 10 mm is associated with greater risk
of type-1 endoleak and stent-graft migration.
Aortic angle is the angle between the flow axis of supra- and infrarenal aortas (aneurysmal
neck), if greater than 60 degrees, this can interfere with the ability of stent-graft
to achieve adequate proximal control.
Aneurysmal neck shape is defined by the difference of the diameter between the proximal
and distal aneurysmal neck. It can be straight, tapered, or reverse tapered. Usually
reverse tapered neck have greater distal aortic neck diameter than proximal one and
a complicated outcome. It also requires more meticulous imaging. Diameters at several
locations should be obtained to assess the extent of tapering within the neck.
For standard EVAR, cylindrical neck of at least 15 mm in length, with a diameter of
33 mm or less and with no or minimal thrombus and calcification is preferred. Excessive
thrombus and extensive calcification may interfere stent-graft apposition against
the aortic luminal surface.
Aneurysm Morphology
Aneurysmal morphology refers to aneurysmal angle, presence of intraluminal thrombus,
and branching vessels from the aneurysm.
Distal Vessel Fixation
Patent, nontortuous, and nonstenotic iliac arteries with distal landing zone of >30 mm
are preferred. Patency of both internal iliac arteries is also important to prevent
pelvic ischemia. The diameters of iliac arteries should be measured at several points
before iliac bifurcation, with special focus on the intended landing zone. The distance
between the aortic bifurcations and iliac bifurcation was recorded on both sides.
Iliac bifurcation and aortic bifurcation diameter should be measured as narrower diameter
can lead to stent-graft occlusion due to unexpected stent-graft kinking. Diameters
of landing zones are measured using the minor axis from the outer to outer (from adventitia
to adventitia) margin, even in the presence of mural thrombus within the arterial
lumen.
Distal Vessel Evaluation
Normal looking, noncalcified common femoral arteries are preferred for smooth stent-graft
delivery.
Current Grafts Used
There are currently six endografts approved by the U.S. Food and Drug Administration
(FDA) for the treatment of infrarenal AAA which are available in the market as given
in [Table 1].
Table 1
Current infrarenal abdominal aneurysms available in the market
|
Device name
|
Company
|
Configuration
|
Maximum device diameter
|
Minimum device diameter
|
Active fixation
|
Anatomical fixation
|
|
Zenith
|
Cook
|
Trimodular
|
36
|
22
|
Suprarenal stent with barbs
|
|
|
Talent
|
Medtronic
|
Bimodular
|
36
|
22
|
Suprarenal stent
|
|
|
Aneurx
|
Medtronic
|
Bimodular
|
28
|
20
|
|
|
|
Endurant
|
Medtronic
|
Bimodular
|
36
|
23
|
Suprarenal stent with barbs
|
|
|
Excluder
|
Gore
|
Bimodular
|
31
|
23
|
Barbs
|
|
|
Powerlink
|
Endologix
|
Unibody
|
34 (Cuff)
|
22
|
Suprarenal stent on aortic cuff
|
Deployment at aortic bifurcation
|
Device Selection
Currently, most used grafts are modular bifurcated grafts. A bifurcated graft has
two components that are inserted separately and then joined. The primary component
consists of an aortic and iliac stent-graft with and an attachment site for the secondar
component which is placed in the contralateral iliac artery (Blim). For challenging
iliac and distal aortic anatomy, tapered aortoiliac devices can be used. According
to the instruction for use (IFU), abdominal stent-grafts, such as Zenith (Cook Medical)
and Talent (Medtronic), have bare-metal struts that extend over the fabric of the
graft to provide suprarenal fixation. These struts provide active fixation in shorter
aortic necks and in reverse taper necks to prevent slippage. They can cause laminar
thrombus disruption in suprarenal aorta and can inhibit access to renal, mesenteric
artery orifices for further procedures. Despite these concerns, they are used safely
with no major adverse effects.
The stent-graft devices must be oversized, 10 to 20%, with respect to proximal aortic
neck diameter to achieve optimal seal. Aortouni-iliac devices are used along with
femorofemoral bypass surgery typically for patients with ipsilateral iliac artery
occlusion or with narrower aortic bifurcation.
Branch Vessel Embolization
Although the effectiveness of preemptive embolization of aortic branches is controversial,
patients with inferior mesenteric artery diameter >3.0 mm or lumbar artery diameter
>2.0 mm can be candidates for this prophylactic embolization to prevent retrograde
aneurysmal sac filling and enlargement.[8]
Endovascular Aortic Aneurysm Repair Procedure
EVAR can be performed in interventional/catheterization suite, conventional OR or
in a hybrid OR. Interventional suite has quality imaging that greatly facilitates
the procedure but may lack the optimum sterilization and lighting facilities available
in a standard operating room.[9] There is a higher risk of wound infection.[10] Whereas in many centers, EVAR is being performed in operation theaters under mobile
C-arm with safety. OR provides better sterile environment, although imaging quality
may be suboptimal to that of an intervention suite.
The hybrid OR with fixed C-arm positioning affords greater flexibility and safety.
Hybrid OR provides both the facilities of excellent imaging and sterile environment.
Preoperative planning is the most important aspect of the procedure, as major part
of the procedure is performed before entering the operating room. Mostly it is done
by specially designed soft wares.
EVAR can either be performed under local, regional, or general anesthesia. For most
operators, general anesthesia provides a more controlled environment.
Although the steps may vary with different devices but following are the common steps
for a standard EVAR in most cases:
After obtaining access of both common femoral arteries (CFAs) either by open or percutaneous
means, sheaths are placed. A full digital subtraction angiogram is obtained to confirm
aneurysmal configuration, aortic neck, and location of the orifices of both renal
arteries. Main body of stent-graft system is deployed from one of the CFA. Through
the other CFA, cannulation of stent-graft limb is done. After successful cannulation,
contralateral limb stent-graft is deployed after confirming internal iliac artery
(IIA) orifice. Ipsilateral limb extension is completed after confirming IIA orifice.
Ballooning with compliant balloon is performed to expand and attach the stent-graft
to the native vessel at both the proximal and distal sites and at the points of graft
overlap. And in the end, completion angiogram is done to find any post-EVAR endoleak
and to confirm the patency of all graft components.
Precise angiographic imaging is necessary throughout the procedure. The C-arm should
be oriented orthogonal to the branch vessel of interest to avoid parallax error. For
example, cranial angulation should be used for the aortic neck, and caudal angulation
to be used to image iliac arteries. Right and left oblique projections should be used
to visualize the renal arteries ostia and the iliac artery bifurcation.
Endovascular Aortic Aneurysm Repair for Poor Access Routes
Selection of primary access route is influenced by vessel diameter, tortuosity, and
atherosclerotic plaque. Unsuitable iliofemoral arterial anatomy predisposes to access
site complications and represent “poor access route” and a relative contraindication
to EVAR. Small, calcified, heavily diseased and tortuous vessels can present a challenge
and potentially preclude EVAR. With better devices and improved experience, EVAR is
now being offered to patients even with poor access. Techniques, such as guidewire
placement, balloon predilation, and pull-through, can be effective in passing stent-graft
through difficult access arteries.
Endovascular Aortic Aneurysm Repair for Ruptured AAA
EVAR is increasingly being offered for patients with rAAA. The potential advantages
include avoidance from consequences of large abdominal incisions, longer operative
time, and consequences of aortic cross-clamping/reperfusion injury.
In 2000, Ohki and Veith reported 10% (2/10) operative mortality for rAAA treated with
EVAR.[11] A meta-analysis in 2008, reported 21% mortality for patients treated with EVAR.[12] The IMPROVE, a randomized control trial, in 2014 did not show superiority of EVAR
in terms of 30-day mortality over OSR. The perioperative mortality was 35% for EVAR
compared with 37% for OSR.[13] This trial did suggest significant reductions in mortality in EVAR performed use
of local anesthesia.[14]
Endovascular Aortic Aneurysm Repair for Juxtarenal Abdominal Aortic Aneurysms
Juxtarenal/pararenal aortic aneurysm involve the visceral segment and these terms
are used to refer to AAA with infrarenal neck less than or equal to 1 cm in length.
When treating patients with inadequate proximal neck and distal zones, it is necessary
to improve the sealing zone of the stent-graft and move it to a healthier portion
of the aorta or the iliac vessels. There is risk of comprising mesenteric and renal
arteries orifices with use of standard EVAR stent-grafts. Approximately, one-third
of patients with AAA are deemed unsuitable for conventional EVAR due to unfavorable
proximal neck anatomy.[15] Advanced EVAR techniques extend the stent-graft proximally while maintaining perfusion
to vital aortic branches. They are divided into fenestrated EVAR (F-EVAR) or snorkel
or chimney EVAR (C-EVAR).[16] In chimney technique, a suprarenal fixation—in a healthy portion of the aorta—can
be achieved by placing two stent-grafts into the renal arteries and landing into the
aortic lumen in parallel with the aortic main body. In snorkel technique, a parallel
graft is placed alongside the main aortic endoprosthesis to maintain flow in a covered
branch vessel. Initially snorkel was done in emergency situations[17] and more of a rescue procedure, since it needs frequent reinterventions. Early success
with the snorkel technique for juxta renal aneurysms has rapidly made it the procedure
of choice for complex EVAR. The flexibility of the technique and lack of requirement
for custom build devices may make this approach more attractive than branched and
fenestrated stent-grafts. This technique is still open to several discussions regarding
long-term durability and the increased risk of type-1 endoleak. Lee et al have reported
results of using “snorkel technique” for juxarenal aneurysms in 56 patients. Technical
success of snorkel placements was 98.2%. Thirty-day mortality was 7.1%. Postoperative
imaging revealed one renal snorkel graft occlusion at 3 months (98.2% overall patency
rate). Seven (25%) early endoleaks were noted in the first follow-up CT. The secondary
intervention rate was 3.6%.[18] In F-EVAR, aneurysmal sac is excluded by keeping patent all abdominal branches,
such as superior mesenteric artery, the renal arteries, and the accessary renal arteries.
By using F-EVAR, a better seal is achieved because the landing zone is secured in
a straight and healthy portion of the aorta.
The Performance of the Chimney Technique for the Treatment of Complex Aortic Pathologies
(PERICLES) registry evaluated 517 patients treated with 898 total stents during chimney
EVAR. The reported technical success was 97.1%. Overall 30-day mortality was 4.9%
and at 3 years was 25.1%. Patency of the chimney grafts was 94.1%. A total of 119
patients in U.S. centers and 398 in European centers were treated during the study
period. U.S. centers preferentially used Zenith (54%) and European centers the Endurant
(62%) as the main body endograft component.[19] F-EVAR needs customized stent-grafts which takes months before use, as they are
specially manufactured from the company based on the individualized patient anatomy.
With proper planning, late failures are becoming rare.[20] Shahverdyan et al have reported experience of 48 F-EVAR procedures using the Anaconda
custom-made device (Vascutek). The primary technical success rate was 94% with three
unsuccessful cannulations of the renovisceral arteries. The 30-day mortality was 4%.
Two occlusions of the right renal stent/artery were detected.[21]
Percutaneous Endovascular Aortic Aneurysm Repair
Conventionally, common femoral arteries (CFAs) are surgically exposed for stent-graft
delivery. Although it is safe and effective approach but can be associated with increased
wound related complications. Percutaneous EVAR (P-EVAR) is an alternative which is
becoming popular as associated with less of these complications.[22] In PEVAR, CFA is punctured under ultrasound guidance and CFAs are closed with suture-mediated
vascular closure devices mostly used in pair.
A large retrospective study showed that PEVAR was associated with shorter operative
time, shorter hospital stays, and fewer wound complications compared with patients
undergoing surgical cut down for exposing CFAs.[23] A randomized controlled trial (RCT) has also showed feasibility of percutaneous
EVAR with acceptable outcomes.[24] CFAs with extensive calcification are associated with more of access-related complications
and avoided. Percutaneous closure is also contraindicated in stenotic or small caliber
femoral artery and in high femoral artery bifurcation.
Complications following Endovascular Aortic Aneurysm Repair
These can be divided into early and late. Early complications (at the time procedure)
of EVAR may include access vessel injury, improper endograft placement, early endoleaks,
ischemic, or systemic complications (such as the postimplantation syndrome). Late
complications include endoleaks, femoral pseudoaneurysm formation, endograft migration,
graft kinking, or occlusion and endograft infection. Meticulous patient selection
and preprocedural workup can reduce the incidence of these complications.[25]
-
Access vessel injury: rarely iliac artery rupture can happen. It needs placement of
a new stent-graft or open repair.
-
Improper stent-graft placement: this can happen if the stent-graft is used outside
of IFU. Its incidence is higher in patients with hostile neck.
-
Postimplantation syndrome: it is defined when there is >38°C temperature, lasting
for more than 1 day along with leukocytosis (WBC >12,000/µL), and a negative blood
culture.
-
Stent-graft migration: it is defined when there is displacement of stent-graft > 5–10 mm
from its original fixation site. It is treated by proximal extension with aortic cuffs,
large explandable stents to augment and endoanchors (for short aortic neck < 10 mm).
-
Endoleaks are the most common complication after EVAR.[26] They are detected in 10 to 45% of treated patients. There are of five different
types. Type-I endoleaks are of implantation type. They occur due to improper apposition
of endograft against the aortic wall and occur either at proximal or at distal implantation
sites. They occur either at proximal (type Ia) or distal (type Ib) sites. Type-II
endoleaks form due to retrograde filling of aneurysmal sac from patent inferior mesenteric
artery (IMA) or from lumbar arteries. Type-III endoleaks form due to defects in the
stent-graft or leakage at the junction points of stent-grafts due to separation of
modular graft components. Type-IV endoleaks are porosity endoleaks. In Type-V endoleaks,
there is gradual increase in aneurysmal sac without evidence of contrast leak. They
are also known as “Endotension.” Types I and III are also known as direct endoleaks,
as there is direct leakage of blood into aneurysmal sac. They can rapidly increase
pressure in the aneurysmal sac and increase risk of aneurysmal growth and rupture.
They need immediate intervention. Types II and IV are known as indirect endoleaks.
Most of them resolve with time. They have more benign course than the direct endoleaks.
Only those patients with increase in aneurysmal size are candidates for intervention.
Endoleaks are also classified as early and late. Early endoleaks can be detected at
the completion angiogram, while late endoleaks are detected on the follow-up visits
either on contrast enhanced or plain CT scan in combination with ultrasound. Contrast-enhanced
CT scan is used because it is more objective. It is very sensitive and specific in
detection of endograft migration, kinking beside the endoleaks. Abdominal duplex ultrasound
(DUS), although operator based, is an alternative in patients with renal dysfunction
who are not candidates for contrast-enhanced CT scan. In experienced hands, DUS can
detect endoleak along with flow direction which are of three different types. The
bidirectional flow is a risk factor for increase in aneurysm diameter.
-
Type 1a endoleaks can be treated by additional stent-graft placements, use of balloon-expandable
metallic stent/aortic cuffs, use of endoanchors, or by embolization with liquid embolic
agents.
-
Type Ib are treated by extension of stent-graft.
-
Type II: only patients with increase in aneurysmal size on serial images needs intervention.
Most can be dealt by angioembolization. Compared with direct endoleaks, type-II endoleaks
are relatively benign. As many as 80% of type-II endoleaks resolve spontaneously within
6 months of the stent-graft implantation. Those that persist are unlikely to cause
aneurysm pressurization, dilatation, or rupture. There is approximately 1% of aneurysm
rupture reported at 2 years in such cases.
-
Type III by placement of additional stent-graft at disconnection or ballooning at
overlap zones to secure modular connection.
-
Type IV are self-limiting and do not need treatment.
Endovascular Aneurysm Sealing
EVAS is a novel technology in which bilateral stents are deployed to create blood
flow lumen to distal anatomy. Endobags filled with biostable polymer create seal.
It is a simple predictable procedure. It reduces type-2 endoleak and secondary procedures.
It can be offered to patients with shorter necks. Studies are required to document
the clinical performance for EVAR and its long-term outcomes.[27]
Post–Endovascular Aortic Aneurysm Repair Rupture
Life-long surveillance and meticulous attention are needed to any possible endograft
malfunction, leading to possible rupture. This has been reported to 1.5% at mean of
29 months.[28]
Disadvantages of Endovascular Aortic Aneurysm Repair
They include life-long surveillance, exposure to radiation, and a higher rate of reintervention
and a higher rate of aneurysmal-related death after 6 months.
